In a variety of AO devices, such as acousto-optic tunable filters (AOTFs), AO modulators, AO Q-switches, deflectors and AO frequency-shifters, a transducer having an electrode thereon (generally referred to as a “top electrode”) on a face of an AO crystal receives a radio frequency (RF) electrical drive signal that results in emanating a forward acoustic wave into the AO crystal that is directed to interact with an optical beam to be processed by the AO device. The AO crystal is conventionally optically smooth on all of its faces, with one approximate criterion for smoothness being the Rayleigh criterion, where a surface is generally considered to be optically smooth if d<λ/(8 cos θ), where d is the surface roughness (e.g., root-mean-square roughness (rms) height measured from a reference plane), λ is being the wavelength of the incidence, and θ being the angle of incidence of the illumination beam. A typical rms roughness for an optically smooth surface for an AO device is generally 10 nanometers, or less.
The acoustic wave after interaction with the optical beam is typically absorbed by an acoustic absorber (or “beam dump”), such as an epoxy layer filled with silver particles, that is typically on the face of the AO crystal substantially opposite the transducer positioned to receive the acoustic beam after its interaction with the optical beam. The acoustic absorber helps prevent backward (180 degree) travelling acoustic waves from also interacting with the optical beam. As known in the art, backward travelling acoustic waves cause problems by Doppler shifting the frequency of the optical beam in the opposite direction relative to the forward acoustic wave Doppler shift of the frequency of the optical beam, which results in the filtered optical beam at the output of the AO device having undesired harmonics of the fundamental frequency of the unfiltered optical beam. The reflected acoustic waves can also cause problems where the AO device is designed to produce a periodic and controllable loss, such as such as when used as a Q-switching element inside a laser cavity. In this case, echoes can cause unwanted bursts of loss to occur after the main burst, and this can lead to a malfunction of the laser system.
The acoustic absorber is known to increase in temperature during operation of the AO device as the acoustic absorber absorbs energy from the arriving acoustic wave. The heating of the acoustic absorber can result in significant heating of the AO crystal particularly for applications that utilize relatively high RF power per unit area of the active transducer.